a very narrow peak of emissions, meaning that the colors emitted by the particles can be precisely controlled;

elimination of a tendency to blink on and off, which limited the usefulness of earlier quantum-dot applications.

Recent research has focused on “the properties we really need to enhance [dots’] application as light emitters, which are the properties that the new results have successfully demonstrated, says Moungi Bawendi, the MIT Lester Wolfe Professor of Chemistry. The new quantum dots, for the first time, he says, “combine all these attributes that people think are important, at the same time.”

Background

Quantum dots — in this case, a specific type called colloidal quantum dots — are nanoscale particles of semiconductor material that are so small that their properties differ from those of the bulk material:

They are governed in part by the laws of quantum mechanics that describe how atoms and subatomic particles behave.

When illuminated with ultraviolet light, the dots fluoresce brightly in a range of colors, determined by the sizes of the particles. They have the potential for many applications, but have faced a series of hurdles to improved performance.

First discovered in the 1980s, these materials have been the focus of intense research because of their potential to provide significant advantages in a wide variety of optical applications and in creating energy-efficient computer and television screens.

While such displays have been produced with existing quantum-dot technology, their performance could be enhanced through the use of dots with precisely controlled colors and higher efficiency.

Multicolored biological dyes

Exposed to ultraviolet light, the dots fluoresce with a color determined by the dot size. A rainbow of colors can be emitted from a single material simply by changing the dot size. (Credit: Drexel University)

One potential application of great interest to researchers is as a substitute for conventional fluorescent dyes used in medical tests and research. Quantum dots could have several advantages over dyes, including the ability to label many kinds of cells and tissues in different colors because of their ability to produce such narrow, precise color variations. But the blinking effect has hindered their use: In fast-moving biological processes, you can sometimes lose track of a single molecule when its attached quantum dot blinks off.

In the new research, blinking was strongly suppressed, meaning the dots stay “on” 94 percent of the time.

High efficiency, small uniform size

The small size of these new dots is important for potential biological applications, explains Bawendi,. “[Our] dots are roughly the size of a protein molecule,” he says. If you want to tag something in a biological system, he says, the tag has got to be small enough so that it doesn’t overwhelm the sample or interfere significantly with its behavior.

‘The new particles were made with a core of semiconductor material (cadmium selenide) and thin shells of a different semiconductor (cadmium sulfide). They demonstrated very high emission efficiency (97 percent) as well as small, uniform size and narrow emission peaks.

Useful applications in two years

A key factor in getting these particles to achieve all the desired characteristics was growing them in solution slowly, so their properties could be more precisely controlled and to allow every atom to go to the right place, said MIT chemistry postdoc Ou Chen.

The slow growth should make it easy to scale up to large production volumes, because it makes it easier to use large containers without losing control over the ultimate sizes of the particles. Chen expects that the first useful applications of this technology could begin to appear within two years.

Taeghwan Hyeon, director of the Center for Nanoparticle Research at Seoul National University in Korea, who was not involved in this research, says, “It is very impressive, because using a seemingly very simple approach — that is, maintaining a slow growth rate — they were able to precisely control shell thickness, enabling them to synthesize highly uniform and small-sized quantum dots.” This work, he says, solves one of the “key challenges” in this field, and “could find biomedical imaging applications, and can be also used for solid-state lighting and displays.”

In addition to Chen and Bawendi, the team included seven other MIT students and postdocs and two researchers from Massachusetts General Hospital and Harvard Medical School. The work was supported by the National Institutes of Health, the Army Research Office through MIT’s Institute for Soldier Nanotechnologies, and by the National Science Foundation through the Collaborative Research in Chemistry Program.